1. Field of the Invention
The present invention relates to a backlight unit. More specifically, the present invention relates to a film-type light guide plate capable of readily securing mass-production and realizing low weight and slimness, a method for manufacturing the same and a backlight unit using the same.
2. Discussion of the Related Art
Cathode ray tubes (CRTs) are general purpose display devices that are used for TVs, measurement instruments, and monitors such as information terminals. However, CRTs cannot actually meet demands for miniaturization and low weight due to their weight and size.
Accordingly, counter to the recent trends toward miniaturization and low weight of electronic products, CRTs have limitations such as weight and size. Thus, liquid crystal displays (LCDs) using electro-optical effects, plasma display panels (PDPs) using gas discharge and electroluminescence displays (ELDs) using electroluminescent effects are potential alternatives to CRTs. Of these, liquid crystal displays (LCDs) are being actively studied.
Liquid crystal displays (LCDs) having advantages including miniaturization, low weight and low power consumption have been actively developed as alternatives for CRTs. LCDs capable of sufficiently serving as flat panel displays are developed and used for laptop computer monitors, desktop computer monitors, large information display devices, etc. Accordingly, demand for liquid crystal displays (LCDs) is gradually increasing.
Most LCDs are light-receiving devices which display an image by controlling an amount of light entering from the outside. A separate light source (i.e., a backlight unit) to irradiate light to an LCD panel is necessarily required.
Generally, a backlight unit used as a light source of liquid crystal displays (LCDs) is a silver cylindrical fluorescent lamp and is classified into an edge-type and a direct-type.
First, direct-type backlight units were the first to be developed, since liquid crystal displays (LCDs) increased in size above 20 inches. Such a direct-type backlight unit directly emits light to the front side of a LCD panel through a plurality of lamps arranged in a row.
An edge-type backlight unit comprises a lamp unit on the side of a light guide plate to guide light, wherein the lamp unit comprises a lamp to emit light; a lamp holder inserted into both ends of the lamp, to protect the lamp; and a lamp reflection plate which surrounds the circumference of the lamp, and has one side inserted into the one side of a light guide plate and thus reflects from a lamp to the light guide plate.
An edge-type backlight unit wherein a lamp unit is arranged on the side of the light guide plate is applied to small liquid crystal displays (LCDs) such as laptop computers or desktop computers and has advantages of uniform distribution of light, long lifecycle and the slimness of liquid crystal displays (LCDs).
Hereinafter, a conventional light guide plate will be illustrated with reference to the annexed drawings.
The light guide plate is classified into two types, i.e., a wedge type and a flat type. For example, FIG. 1A illustrates a wedge-type light guide plate including a light-receiving portion and a non-light-receiving portion having different thicknesses and FIG. 1B illustrates a flat-type light guide plate having a light-receiving portion having the same thickness.
FIG. 2 is a schematic view illustrating a light-emission principle of a general light guide plate.
As shown in FIG. 2, a light guide plate 10 is arranged on the side of a lamp 5, and a protrusion pattern 10a is arranged under the light guide plate 10. The light guide plate 10 is made of a material having a refractive index higher than that of air. For light-emission of the light guide plate, at the interface between air and the light guide plate 10, when transmitting from a high refractive index material to a low refractive index material, light is not refracted and total reflection occurs on the interface in accordance with Snell's law. When a pattern is absent, the light entering the light guide plate 10 through total-reflection can be transmitted to the edge. Accordingly, in order to transmit guided light to the outside of the light guide plate 10, light is refracted through the protrusion pattern 10a at an angle to prevent total-reflection through the protrusion pattern 10a and transfers above the light guide plate 10. Light is emitted from regions where the protrusion patterns 10a are present in proportion to the size of the protrusion patterns 10a and then transfers to the top of the light guide plate 10. The protrusion patterns 10a may be different in respective regions in order to emit totally uniform light.
The light guide plate 10 is generally formed by injection molding. The formation process will be illustrated with reference to the annexed drawings.
FIG. 3 is a schematic view illustrating an injection molding apparatus used for formation of a light guide plate.
As shown in FIG. 3, an injection molding apparatus used for the formation of the light guide plate comprises a cavity portion 30 to define the shape of the light guide plate, and a molten resin-injection portion 40 connected to the cavity portion 30, which transmits a thermoplastic resin to the cavity portion 30 and controls flow of the resin.
The cavity portion 30 includes a first mold 31 and a second mold 32 that may be separated from each other, and a hole 35 provided between the first and second molds 31 and 32, when the first and second molds 31 and 32 come into contact with each other. The shape of the hole 35 corresponds to the desired shape of the light guide plate.
A gate 44 is provided between the cavity portion 30 and the molten resin injection portion 40 and allows a thermoplastic resin to flow through the molten resin injection portion 40.
The molten resin injection portion 40 includes an injection cylinder 42 to store the thermoplastic resin 41 and a screw 43 to control discharge of the thermoplastic resin 41 from the cylinder 42.
The injection molding using the injection molding apparatus comprises tightening the first and second molds 31 and 32 of the cavity portion 30, injecting a liquid form of the molten resin 41 through the gate 44 into the hole 35 corresponding to the desired shape of the light guide plate provided between the molds, performing injection molding at high pressure and high pressure, and cooling and curing the thermoplastic resin into the hole 35 to form a light guide plate. The method further comprises opening the first and second molds 31 and 32 to separate the light guide plate formed by assembling the molds 31 and 32 therefrom, thereby completing the formation.
FIGS. 4A and 4B are vertical-sectional views illustrating an example of the injection-molded light guide plate.
As shown in FIGS. 4A and 4B, the light guide plate separated from the cavity portion 30 is injection-molded, cured and then left in the form of a protrusion pattern together with the thermoplastic resin left in the region corresponding to the gate 44.
In this case, the protrusion pattern should be removed through an additional post-process such as cutting, or grinding.
FIG. 5 is a schematic view illustrating a conventional cavity portion comprising a mold for formation of a light guide plate.
As shown in FIG. 5, for the conventional injection molding apparatus for formation of light guide plates, the cavity portion shown in FIG. 3 comprises a main core 62 including molds spaced from each other, and a stamper pattern 61 corresponding to a pattern 61a present on the surface of the hole 35 provided between the spaced molds.
The formation of the light guide plate requires manufacturing processes for the main core 62 and the stamper pattern 61.
The mold of the main core 62 determines the type of the light guide plate, i.e., a wedge or flat type. An inclined pattern 63 is provided in the main core 62 and omitted when the light guide plate is a flat type.
The formation of the main core 62 is basically carried out by body formation, surface coating and side grinding (wedge type) in this order.
The shape of the stamper pattern 61 determines a dispersion pattern density on the surface of the light guide plate. The manufacturing of the stamper pattern 62 is carried out by forming a bite for processing the pattern, processing a stamper pattern and forming a stamper pattern coating film.
As such, after manufacture of the main core 62 and the stamper pattern 61, molds spaced from each other are finally assembled to complete the cavity portion 30.